This invention relates generally to the recovery of marketable products such as oil and gas from substantially fluid impermeable deposits of viscous hydrocarbonaceous liquid in an inorganic matrix such as tar sand, by the application of electromagnetic energy to heat the deposits. More specifically, the invention relates to a method for recovering hydrocarbonaceous liquids from such formations by controlled electromagnetic heating to vaporize water therein to drive out such liquids, while controlling the electromagnetic power to limit the vaporization of water to control the resulting steam drive. The invention relates particularly to such method including use of a high power radio frequency signal generator and an arrangement of elongated electrodes inserted in the earth formations for applying electromagnetic energy to provide controlled heating of the formations.
Vast amounts of hydrocarbons are contained in deposits from which they cannot be produced by conventional oil production techniques because the hydrocarbons are too viscous and the formations are substantially fluid impermeable. Such deposits include the Utah tar sand deposits estimated to contain 26 billion barrels of bitumen. They include enormous tar sand deposits in Western Canada and other deposits of viscous oils.
It is well known to mine tar sands, heating the mined tar sands on the surface of the earth to an appropriate temperature in the presence of aqueous surfactant solutions, and recovering the products thereupon released from the matrix. In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is relatively large, since bitumen typically constitutes only about ten percent of the total by weight. Material handling of tar sands is particularly difficult even under the best of conditions, and the problems of waste disposal are substantial.
A number of proposals have been made for in situ methods of processing and recovering valuable products from hydrocarbonaceous deposits. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Valuable constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injecting another substance into the formation. It is important to the success of such methods that the amount of energy required to effect the extraction be minimized.
It has been known to heat relatively large volumes of hydrocarbonaceous formations in situ using radio frequency energy. This is disclosed in Bridges and Taflove U.S. Pat. No. Re. 30,738. That patent discloses a system and method for in situ heat processing of hydrocarbonaceous earth formations wherein a plurality of conductive means are inserted in the formations and bound a particular volume of the formations. As used therein, the term "bounding a particular volume" was intended to mean that the volume was enclosed on at least two sides thereof. In the most practical implementations, the enclosed sides were enclosed in an electrical sense, and the conductors forming a particular side could be an array of spaced conductors. Electrical excitation means were provided for establishing alternating electric fields in the volume. The frequency of the excitation means was selected as a function of the dimensions of the bounded volume so as to establish a substantially non-radiating electric field which was substantially confined in such volume. In this manner, volumetric heating of the formations occurred to effect approximately uniform heating of the volumes.
In an embodiment of the system described in that patent as applied to tar sands, the frequency of the excitation was chosen to assure adequate absorption for uniform heating while being sufficiently low to prevent radiation. In that embodiment, the conductive means comprised conductors disposed in respective opposing spaced rows of boreholes in the formations. One structure employed three spaced rows of conductors which formed a triplate type of waveguide structure. The stated excitation was applied as a voltage, for example, between difficult groups of the conductive means or as a dipole source, or as a current which excited at least one current loop in the volume. Particularly as the energy was coupled to the formations from electric fields created between respective conductors, such conductors were, and are, often referred to as electrodes.
Materials such as viscous oils and tar sands are amenable to heat processing to produce gases and hydrocarbonaceous liquids. Generally, the heat develops the permeability and/or mobility necessary for recovery. Tar sands is an erratic mixture of sand, water and bitumen with the bitumen typically present as a film around water-enveloped sand particles. Using various types of heat processing, the bitumen can be separated.